1. Field of the Invention
The present invention relates to an apparatus for detecting truss plates. More specifically, this invention relates to a detector that identifies missing or misaligned truss plates during the truss assembly process.
2. Background Art
One of the most problematic and recurring quality control issues in a truss mill is that of missing truss (e.g., nail) plates on finished products. The issue exists in nearly all truss mills, whether they are building custom trusses for an expensive home, or simply building the same stock trusses for local lumber yards for days at a time. This problem is common throughout the industry and can be expensive to resolve.
During the truss assembly process, builders tap nail plates into place on the top of each lumber component intersection (joint). The builder then pries up the truss to place a companion plate in the same position on the back side. A roller gantry moves down the line and forces each plate partially into the lumber. The truss is then moved off to a final exit roller (press) which is generally set to an exact 1.5″ span. The exit roller presses with enough force to embed the nail plates into the truss and provide the strength that the truss was engineered to have. The truss is then moved down a line and stacked up with other trusses in a job. When the job stack is complete, it is banded together and placed on a truck to be delivered to the customer (e.g., a building contractor).
Unfortunately, however, the roller gantry system requires two elements for ideal operation. The first requirement is an attentive builder. If a builder is not fully attentive, or is rushing through production to meet quotas, it is very easy to miss positioning a nail plate, particularly on the bottom side of the truss. Omitting nail plates on the bottom of the truss is a common mistake. The builder must also remain attentive after the truss is complete and is being pressed by the exit rollers. If a truss is missing a nail plate, the builder should catch the error before the truss is stacked with the other trusses. If the builder fails to notice the missing plate, the error will likely not be caught until the truss has already been shipped to the job site.
The second important element for ideal operation of the roller gantry is mechanical accuracy of the equipment. If gantry tables are misaligned or out of level, the rolling press action that embeds the nail plates partially into the lumber can be inefficient. As a result, when the truss is popped up to be sent through the exit roller, nail plates may only be loosely connected on a back side of the truss or may not be connected at all. Another problem can be improperly operating conveyance rollers that are used to move the truss to the exit rollers. Bad or damaged rollers can catch poorly embedded nail plates and cause them to fall off or be peeled back and folded up prior to reaching the exit roller press. Either error results in a truss that cannot meet the engineered joint strengths.
Generally, when a truss leaves the mill without proper plating, it is the contractor or end customer that finds the defect and reports it to the truss mill management. Since the improperly plated truss does not meet the required engineering standards for the truss design, it is the truss mill's responsibility to remedy the situation. This can be done by sending a mill employee to the job site with the required nail plate(s) and some special field equipment that allows the employee to press the plate into the lumber with the necessary pressure. The other solution would be to set up and build an entirely new truss to be delivered to the job site. Neither of these solutions are desirable, however, particularly considering that most truss mills deliver their products within a radius of about 100 miles, and sometimes up to 200 miles or more.
The time and expense of sending a replacement truss, or an agent and equipment to repair the truss, can be quite substantial, eating into the truss mill's profit. In addition, negative public relations can result, particularly if the contractor is delayed substantially while waiting for the truss to be repaired, or if missing nail plate mistakes happen frequently. Contractors and customers view this problem as a serious lack of quality control and may consider looking to other sources to fulfill their future truss requirements.
The industry would be benefited by a method and apparatus for monitoring trusses during the production process to detect missing plates before they are placed in a stack and shipped off to a job site. The industry would also benefit from a method and apparatus that can detect misaligned nail plates.